<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>

    </style>
</head>
<body>
    <div>
        <input type="text" id="input">
        <input type="button" id="code" onclick="createCode()">
        <input type="button" value="验证" onclick="validate()">
    </div>
    <script>
        // 在全局定义验证码
        var code;
        window.onload=function createCode(){
            code="";
            // 验证码长度
            var codeLength=4;
                        var checkCode = document.getElementById("code");   
            var random = new Array(0,1,2,3,4,5,6,7,8,9,'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R',  
            'S','T','U','V','W','X','Y','Z');//随机数  
            for(var i = 0; i < codeLength; i++) {//循环操作  
                var index = Math.floor(Math.random()*36);//取得随机数的索引（0~35）  
                code += random[index];//根据索引取得随机数加到code上  
            }  
            checkCode.value = code;//把code值赋给验证码  
            }  
            //校验验证码  
            function validate(){  
            var inputCode = document.getElementById("input").value.toUpperCase(); //取得输入的验证码并转化为大写     
            if(inputCode.length <= 0) { //若输入的验证码长度为0  
                alert("Empty Code!"); //则弹出请输入验证码  
            }      
            else if(inputCode != code ) { //若输入的验证码与产生的验证码不一致时  
                alert("Error Code"); //则弹出验证码输入错误  
                createCode();//刷新验证码  
                document.getElementById("input").value = "";//清空文本框  
            }      
            else { //输入正确时  
                alert("OK"); //弹出^-^  
            }        
            }  
    </script>
</body>
</html>